A great deal of work in sentence processing over the years has dealt with the question of whether contextual information can guide language processing. There is by now considerable evidence that suggests that some kinds of information from the discourse context have immediate effects. However, the question of how these effects occur has remained largely ignored. The studies in this proposal focus on the discourse properties of modified definite noun phrases. Work in sentence processing has shown that the resolution of ambiguities in which one of the possible readings involves noun modification is affected by the availability of a discourse model in which the modificational phrase serves to distinguish between two possible referents. A central question is whether such discourse effects found with modifiers reflect a general, conventionalized property of modification, or whether they are more aptly characterized as a more subtle system based on expectations regarding typical usages. A series of studies is proposed to investigate the hypothesis that a typical default expression exists for neutral (i.e., non-contrastive) contexts, and that the use of a more informative expression signals a contrastive function in the discourse, with immediate processing consequences. Data will come from elicited production tasks, on-line comprehension experiments which monitor subjects' eye movements to a visual array in response to spoken linguistic stimuli, traditional reading time studies, and prosodic analyses in a read-aloud task. The current proposal represents a significant departure from existing work in two salient ways: First, it attempts to provide a detailed investigation into the nature of the referential effects, using a methodology that is especially well-suited for studying referential aspects of language. Second, whereas previous findings have been couched almost exclusively in terms of the mechanisms of sentence processing, the current proposal seeks to integrate experiments from on-line language processing and language production. Results of this project may be useful in developing models for language disorders, for the development of pedagogical tools, and for progress in artificial intelligence.